1. Field of the Invention
The present invention relates to a superconducting member. More particularly, the present invention relates to a superconducting member comprising a super conducting material in a part of a component which can transmit a high frequency electric power having a predetermined frequency on the order of MHz or more selectively and with low loss.
2. Background Art
Superconducting filters can realize the production of a small-size resonator having a small material loss and a high Q value. Consequently, steep skirt properties unattainable by the conventional filter can be realized, and, hence, efforts have been made to put high frequency filters on the order of MHz or more to practical use. Among others, ReBa2Cu3O7-δ, wherein Re represents not more than three elements selected from lanthanum (La), yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), dysprosium (Dy), ytterbium (Yb), neodymium (Nd), holmium (Ho), and erbium (Er) (hereinafter referred to as “(Re)BCO”), which has a superconducting transition temperature of about 90 K and a high critical current value, is expected as a promising material.
Sapphire substrates are considered as optimal substrates for the preparation of high frequency filter circuits, for example, from the viewpoints of relatively low price as a large-area single crystal substrate, a suitable permittivity necessary for the preparation of high frequency circuits on the order of MHz or more, high mechanical strength, and large low-temperature thermal conductivity.
However, it is very difficult to form an (Re)BCO film having good superconducting properties directly on the sapphire substrate, and, so far as the present inventors know, there is no report about any example of success in forming such films. At the present stage, a method has been extensively adopted as a solution to the problem in which a buffer layer is first grown on a sapphire substrate and a thin superconducting film is then grown on the buffer layer. When this method is applied, due to lattice matching between the superconducting material and the material constituting the buffer layer, it is common practice to form a buffer layer having a lattice constant close to that of the superconducting material, for example, ceria or YSZ, on an R face (1102) or an A face (1120) of the sapphire substrate and then to form a thin (Re)BCO film on the buffer layer.
In order to improve the properties of a thin superconducting film formed on the upper part of the buffer layer, the buffer layer used in this method has hitherto been required not to sacrifice the crystallinity of the superconductor used.
To meet this demand, studies have been forwarded with a view to maximizing the flatness of the buffer layer and maximizing the oblateness of the grain lumps. This is because the buffer layer assumes the superconducting properties of the (Re)BCO super conductor. It is considered that the superconducting properties are improved with reducing the concavoconvexes of a CuO two-dimensional network structure formed of Cu atoms and O atoms.
In fact, it is known that, due to the nature of the crystal structure, mercury(Hg)-type copper oxides having small concavoconvexes in the CuO network structure is unsuitable for practical use because of their many problems with raw materials and production process used, but on the other hand, by virtue of good superconducting properties, the superconducting transition temperature is high and 100 K or above.
Any technique for accurately regulating the surface shape of the buffer layer provided on the sapphire substrate has not been established yet. A current main recognition on the thin superconducting film formation is that the formation of a flat buffer layer having a flatter surface shape can realize the formation of a thin superconducting film having better crystallinity which in its turn can provide a thin superconducting film having excellent superconducting properties. Accordingly, at the present time, the development of a method for forming a flatter surface shape as the buffer layer for a thin superconducting film is forwarded.
However, it is very difficult to realize an (Re)BCO film having a large thickness of not less than 300 nm and simultaneously possessing excellent crystallinity and excellent superconducting properties. The reason for this is considered attributable to the influence of internal strain of crystal constituting the film which increases with increasing the thickness of the film, but any apparent reason has not been elucidated yet.
Further, it should be noted that, when an (Re)BCO film having a thickness of not less than 300 nm is formed on the surface of the sapphire substrate, the realization of a critical current density at a liquid nitrogen temperature (hereinafter abbreviated to “Jc”) of not less than 4.0×106 A/cm2 is difficult. For this reason, a satisfactorily large critical current value (hereinafter referred to as “Ic”) necessary for applications in transmission in high frequency filters, or applications in electric power where a large current should be allowed to flow into a superconductor has not been realized.
Accordingly, at the present time, studies on the preparation of a film having a large thickness of not less than 1 μm by providing an atom step on the substrate surface to relax the crystal strain of the thin film are forwarded. When this method as such is used, a thick film having a thickness of about 1 μm which exhibits superconducting properties can be prepared, but, when a sapphire substrate is used, it is difficult to provide an Ic (=Jc×film thickness) value of not less than 250 A/cm at a liquid nitrogen temperature. Further, when this method is applied to form a superconducting layer or a buffer layer on the upper part of an R face sapphire substrate with concavoconvexes having a size of not less than 5 nm, the crystallinity of the superconducting layer and the buffer layer cannot be maintained, making it impossible to provide good superconducting properties in the superconducting layer provided on the buffer layer, not to mention in the superconducting layer provided directly on the substrate.
Accordingly, at the present time, in order to prepare a thick superconducting film having excellent superconducting properties, it is necessary to clarify that which shape is required in the surface structure of the buffer layer as a base for the superconducting film. Up to now, however, since any technique for accurately regulating the surface shape of the buffer layer provided on the sapphire substrate has not been established, it has been very difficult to examine the surface shape of the buffer layer necessary for the superconducting layer provided on the buffer layer.